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Comments on High pass filter design

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High pass filter design

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Despite there are many filter calculators available on the web, I still have difficulties to translate what I need into calculator design parameters.

For my application, I need ideally a high pass filter that kill everything below 60Hz, and keep everything near 100Hz. More mathematically, I would like a HP filter that keep 99% of the signal (voltage) at 90 Hz, and kill x % of the signal at 60Hz (say x = 98%, but I can compromise to reduce the complexity of the filter).

Other considerations are:

  1. I don't care of phase distortion
  2. active filter is OK
  3. max amplitude of the signal +/- 8V
  4. very weak current (it is to be the input of an oamp).

How the real pros would translate these needs into design rules for a calculator?

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How the real pros would translate these needs into design rules for a calculator?

Speaking as a pro I wouldn't use a calculator because I'd use a simulator and tinker with a few values until I got the response I wanted. I'd probably use standard building blocks like high-pass sallen key filters and cascade them until I got the desired response.

I might also consider using a notch filter to remove the problematic frequency area sufficiently.

I'd also be concerned about the filter circuit adding noise to what might be a very small input signal.

Bottom line: I would simulate to find the right type of filter that fits the requirements and then do calculations on what unwanted noise that filter might bring to the party.

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coquelicot‭ wrote about 4 years ago

Thank you for your answer Andy. Curiously, this is more or less what I tried to do. But I think this time, Olin was right: there is a limit of what is possible to do with analog electronics: Even a 9th order elliptic filter would not give me the required spec. Only a Fourier transform performed on a 100 pt curve will do. The other solution is to modify my probe to get 2 identical signals with a phase offset of 180 deg and to kill the common mode pickup by subtracting the signals.